Precision instruments are incorporated with such devices as semiconductor integrated circuits which are produced by the process involving optical lithography or nano-imprinting. This process is strongly required to yield a substrate free of defects as much as possible on its surface. Any defect on a masking substrate, for example, to be used as an original plate for exposure in photolithography, would be transferred to cause defective patterning.
The above-mentioned substrate varies in thickness depending on its uses. Usually, it may be as thin as about 0.1 to 1.0 mm if it is intended for complementary metal-oxide semiconductor (CMOS) sensors, light waveguide sensors, sequencer chips, etc. Such thin substrates, glasses used in DNA sequencer chips for example, are conventionally produced by forming a pattern on one side of a thick substrate and grinding the reverse side subsequently. This conventional process consisting of patterning and ensuing grinding has a disadvantage that the substrate is liable to break as the patterning becomes more intricate than before with generation change. One way under development to overcome this disadvantage is by patterning on an originally thin substrate.
Production of those substrates for photomasks and optical sensors usually involves several steps, such as lapping and polishing, to prepare the surface with a high degree of flatness and a minimum of defects.
A substrate of synthetic quartz glass suffers impact during lapping and polishing, with the magnitude of impact varying between them. Lapping gives rise to a much more significant impact than polishing, because lapping usually employs a lapping plate of cast iron, which is much harder than the substrate, and the lapping plate comes into direct contact with the substrate. By contrast, polishing only causes insignificant impact, because polishing employs a polishing cloth, which is softer than the substrate.
In order to minimize defects caused by impact on the substrate surface, there have been proposed several methods for applying the lapping plate onto the substrate in the lapping step.
An example of such methods, disclosed in JP-A 2012-192486 (Patent Document 1), consists of pressing the polishing plate against the substrate by means of hydraulic pressure instead of conventional pneumatic pressure. The hydraulic pressure is exempted from the irregular pressure fluctuation characteristic of air stream, and hence it does not apply any uneven pressure to the substrate when the polishing plate comes into contact with the substrate and while polishing is performed. Thus, the disclosed method permits any thin workpiece to be fabricated without damage.
Another example of methods, disclosed in JP-A H09-109021 (Patent Document 2), consists of employing a suction pad that levels warpage on the substrate surface to be polished, and also employing a highly rigid polishing plate with minimum warpage, thereby keeping uniform the pressure applied to the surface being polished. Thus, the disclosed method is able to prevent uneven pressure from occurring in the substrate surface when the polishing plate comes into contact with the substrate and while polishing is performed.